Acoustic sensor and array thereof

ABSTRACT

The present invention provides an acoustic sensor having one or more segments that are electrically coupled to provide a response corresponding to a hydrodynamic pressure applied to the segments. Each segment contains a substrate of a desired shape with a concavity on an outer surface that is sealingly enclosed by an active member made from a flexible, resilient piezoelectric material. PVDF material is preferably used as the piezoelectric active element. The active element may be bonded to a compliant diaphragm sealed to the substrate to provide the sealed chamber. The active member is covered with a protective layer of a suitable material, preferably a polyvinyl material. In one embodiment, the diaphragm includes a standoff ledge and is placed on the outer surface of the substrate to define the sealed chamber between the diaphragm and the outer surface of the substrate. In another embodiment, at least two substrates are used and a damping material is placed between the substrates wherein one substrate includes a concavity on an outer surface for defining the sealed chamber. In still another embodiment, the diaphragm having the standoff ledge is used with the at least two substrates.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. Pat. Application Ser.No. 08/573,009, now U.S. Pat. No. 5,774,423 entitled "Acoustic Sensorand Array Thereof" (Richard E. Pearce, et al), filed Dec. 15, 1995, andis assigned to the same assignee as the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to the field of acoustic sensors andmore particularly to a novel hydrophone and method of making the samewhich may be used under great hydrostatic pressure and under severehydrodynamic conditions.

2. Description of the Related Art

Piezoelectric hydrophones of various configurations have been used in avariety of applications. In geophysical exploration, arrays ofhydrophones are used to detect seismic shock waves from the earth'ssubstrata in response to induced shock waves at known locations on theearth. Hydrophones also are used in boreholes to conduct verticalseismic surveys and for a variety of other applications. Acousticpressure variations across the hydrophone produce electrical signalsrepresentative of the acoustic pressure, which are processed for desiredapplications.

Piezoelectric hydrophones typically contain a piezoelectric material asan active element which produces electrical signals when subjected toacoustic pressures. Ceramic materials such as barium titane or leadzirconate titane have been used in various configurations as one classof piezoelectric materials in hydrophones. U.S. Pat. No. 4,092,628discloses the use of a thin ceramic wafer that operates in the bendermode. U.S. Pat. No. 4,525,645 shows a unit shaped as a right cylinderthat operates in the radial mode. Ceramic materials are brittle and tendto shatter in the presence of a severe shock such as that produced by anexplosive charge or air gun commonly employed for conducting seismicsurveys over water-covered areas.

Most hydrophones have a depth limit. An excessive overpressure can causethe active element to bend beyond its elastic limit, resulting in signaldistortion and ultimate failure of the hydrophone. In the ceramic wafertype hydrophones, an internal stop is sometimes provided to preventexcessive bending of the element. The wafer, however, tends to develop apermanent deformation that degrades the output signal.

Polyvinylidene fluoride ("PVDF") has been used as another class ofpiezoelectric material in hydrophones. One such material is availableunder the tradename KYNAR from AMP corporation. The PVDF material isuseful as a hydrophone active element because its acoustic impedance isclose to that of water and the acoustic wavefields do not producespurious reflections and diffractions as they do when encounteringceramic piezoelectric elements. The output signals of the PVDF elementare many times greater than the signal output of a ceramic material.Also, PVDF material is readily available in various sheet sizes and awide range of thickness. Such PVDF material may be readily shaped andcut to fit the intended use. Prior to use, the PVDF material is poled oractivated in the thickness direction by application of a high electricfield at an elevated temperature for a requisite time period. Conductivemetal electrodes are evaporated on the opposite sides of the PVDF filmas with the ceramic materials.

An external mechanical force applied to the PVDF film results in acompressive or tensile force strain. The PVDF film develops an opencircuit voltage (electrical charge) proportional to the changes in themechanical stress or strain. The charge developed diminishes with time,depending upon the dielectric constant of the film and the impedance ofthe connected circuitry. By convention, the polarization axis is thethickness axis. Tensile stress may take place along either thelongitudinal axis or the width axis.

U.S. Pat. No. 4,653,036 teaches the use of a PVDF membrane stretchedover a hoop ring. A metallic backing is attached to the back of the ringand a void between the film and the backing is filled with an elastomersuch as silicone. The device operates in the bender mode. U.S. Pat. No.4,789,971 shows the use of a voided slab of PVDF material sandwichedbetween a pair of electrodes. A bilaminar construction is alsodisclosed. A preamplifier is included in the assembly. The transduceroperates in the thickness-compressive mode.

A hydrophone array shown in U.S. Pat. No. 4,805,157 consists of multiplelayers of PVDF material symmetrically disposed around a stiffener forprevention of flexural stresses. The axis of maximum sensitivity is inthe direction transverse to the plane of the layers. This sensor issensitive to compressive stress.

U.S. Pat. No. 5,361,240, issued to the inventor of this application,discloses a pressure-compensated PVDF hydrophone that contains a hollowmandrel having a concavity at an outer surface. A flexible and resilientpiezoelectric film, preferably made from a PVDF material, is wrappedseveral times around the mandrel over the concavity to act as the activeelement of the hydrophone. The volume between the surface of the innerlayer of the film and the concavity provides a pressure compressionchamber. This hydrophone has been found to be responsive to varyinghydrodynamic pressure fields but is substantially insensitive toacceleration forces, localized impacts and variations in hydrostaticpressures.

To perform seismic surveys in water-covered areas, one or more arrays ofhydrophones, each array having a plurality of serially coupledhydrophones, are deployed on the bottom of a water-covered area or aretowed behind a vessel. In bottom cable applications, hydrophones arecommonly built as an integral part of the cable. Each hydrophone ishermetically sealed with a suitable material, such as polyurethane. Suchcable constructions are not conducive to easy repairs in the field.Defective hydrophone sections are removed and a cable section containinga working hydrophone is spliced in the place of the defectivehydrophone. Such repairs are usually less reliable than unitaryconstructions and require excessive repair time, which can significantlyincrease the cost of the surveying operations, especially whenperforming three-dimensional seismic surveys as the down time can costseveral thousand dollars per hour. Thus, there has been an unfilled needto provide a hydrophone which is easy to assemble into a hydrophonecable and easy to repair in the field.

The present invention addresses the above-noted problems and provides asegmented hydrophone that preferably utilizes a flexible and resilientpiezoelectric material and a method of making same. The hydrophonesegments may be combined to form a single hydrophone. The hydrophonesegments removably attach to the cable which is suitably configured toaccommodate the hydrophone segments. Any hydrophone segment can readilybe replaced without requiring any splicing of the cable. The hydrophoneis responsive to varying hydrodynamic pressure fields, but issubstantially inert to acceleration forces, localized impacts andvariations in hydrostatic pressure.

SUMMARY OF THE INVENTION

The present invention provides an acoustic sensor having one or moresegments that are electrically coupled to provide a responsecorresponding to an acoustic pressure applied to the segments. Eachsegment contains at least one substrate of a desired shape with achamber on an outer surface that is sealingly enclosed by an activemember made from a flexible, resilient piezoelectric material.Polyvinylidene fluoride material is preferably used as the piezoelectricactive element. The active element is preferably bonded to a compliantdiaphragm sealed to the substrate to provide the sealed chamber. Theactive element is covered with a protective layer of a suitablematerial, preferably a polymer material. A single segment may be used asa sensor or a plurality of segments may be coupled to form the sensor.The sensor of the present invention may be used in hydrophone cables forperforming seismic surveys. In such applications, a plurality of spacedsensors made according to the present invention are attached along thelength of a suitably configured cable. Each such sensor preferablycontains two electrically coupled hydrophone segments are placed on arigid member placed on the cable. One or both hydrophone segments arecoupled to a conductor in the cable, preferably via an under waterplug-in connector. The cable is configured to accommodate the hydrophonesegments between a nose section and a rear or tail section. One or moreof such hydrophone cables are usually arranged in a matrix or an arrayfor performing seismic surveys. The sensor of the present invention,however, may also be used in other applications requiring the use of ahydrophone.

In one embodiment, the diaphragm has a standoff ledge and is placed onthe outer surface of the substrate to define a sealed chamber betweenthe diaphragm and the outer surface of the substrate. The piezoelectricmaterial is placed over the diaphragm. In a second embodiment, at leasttwo substrates are provided and a damping material is placed between theouter surface of a first substrate and the inner surface of a secondsubstrate. The second substrate includes a concavity on an outer surfacewherein a diaphragm is placed over the concavity of the second substrateto define a sealed chamber between the diaphragm and the concavity. Thepiezoelectric material is placed over the diaphragm. In a thirdembodiment, at least two substrates are provided. The diaphragm includesthe standoff ledge and is placed on the outer surface of a secondsubstrate to define a sealed chamber between the diaphragm having thestandoff ledge and the outer surface of the second substrate. A dampingmaterial is placed between an outer surface of a first substrate and aninner surface of the second substrate.

Examples of the more important features of the invention thus have beensummarized rather broadly in order that the detailed description thereofthat follows may be better understood and in order that thecontributions to the art may be appreciated. There are, of course,additional features of the invention that will be described hereinafterand which will form the subject of the claims appended hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed understanding of the present invention, references shouldbe made to the following detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings, inwhich like elements have been given like numerals and wherein:

FIG. 1 shows a partial sectional view of a two-segment hydrophonecoupled to a cable according to the present invention.

FIG. 1A shows a cross-sectional view of a first embodiment of thehydrophone shown in FIG. 1 taken along A--A.

FIG. 1B shows a cross-sectional view of a second embodiment of thehydrophone shown in FIG. 1 taken along A--A.

FIG. 1C shows a cross-sectional view of a third embodiment of thehydrophone shown in FIG. 1 taken along A--A.

FIG. 2 shows an exploded perspective view of certain elements of thehydrophone of FIG. 1.

FIG. 3 shows a diaphragm having a standoff ledge as illustrated in FIGS.1A and 1C.

FIG. 4 shows a top view of segment 30 of the hydrophone shown in FIG. 1having a plug-in connector for electrically coupling such segment to thecable.

FIG. 5A shows a sectional view of the first embodiment of the hydrophonesegment shown in FIG. 4 taken along B--B.

FIG. 5B shows a sectional view of the second embodiment of thehydrophone segment shown in FIG. 4 taken along B--B.

FIG. 5C shows a sectional view of the third embodiment of the hydrophonesegment shown in FIG. 4 taken along B--B.

FIG. 6 shows a sectional view of the diaphragm of FIG. 3 including aplurality of convex ribs longitudinally positioned on an inner surfaceof the diaphragm.

FIG. 7 shows a top view of an active element for use in the hydrophonesegments of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

For convenience and simplicity and not as a limitation, the sensor ofthe present invention is described as a two-segment hydrophone placed ona type of cable typically used for conducting seismic surveys forgeophysical prospecting. Accordingly, FIG. 1 shows a partialcross-sectional view the preferred embodiment of a two-segmenthydrophone coupled to a cable according to the present invention. FIG.1A, FIG. 1B, and FIG. 1C are cross-sectional views of the hydrophoneassembly of FIG. 1 taken along A--A shown therein each showing adifferent embodiment of the hydrophone assembly. FIG. 2 shows anexploded perspective view of the hydrophone assembly shown in FIG. 1.

Now referring to FIGS. 1, 1A-1C, and 2, the hydrophone assembly includesa cable 10 having a plurality of twisted pairs of conductors 8 placedaround a core member 9 along the cable length. The conductors 8 areencased in one or more layers 12 of protective coatings. A moldedsection 14 is formed over the cable 10 to suitably accommodatehydrophone segments between a nose (front end) 34 having a smoothlyincreasing cross-section from a point of attachment 33 on the cable 10to a point 38 and a smoothly decreasing cross-section tail (rear end)14a that extends from a point 20 of maximal diameter d₁ to a point 18 onthe cable exterior. The front end 34 preferably has a higher slope thanthe tail end 14a to reduce the noise effect due to hydrodynamicturbulence produced when the cable 10 is pulled under water.Longitudinal flanges 22a and 22b, each having a desired width d₂ extendsaxially along the cable 10 from the maximal diameter point 20 up to thepoint 38 on opposite sides of the cable 10. Each flange contains aplurality of holes 24 for accommodating therein suitable tying elementssuch as bolts. Conductors 16 taken out from the cable are coupled to asuitable underwater connector 17 for providing electrical connectionbetween the cable 10 and the hydrophone segments. The connector 17 andthe conductors 16 are preferably molded in the section 14a exposing onlythe mating end of the connector 17 to the end 20.

Hydrophone segments 30 and 32, each having an inner surface thatsubstantially conforms to the outer surface of suitable rigid membersplaced on the cable, are placed over the rigid member juxtaposed withthe edge 20. The rigid members provide support for the hydrophonesegments and prevent bending of the hydrophones when the cable 10 bendsduring handling and use. The hydrophone segments 30 and 32 may beconveniently secured on the cable by attaching the segments to theflanges 22 by bolts 26 placed through the holes 24 and correspondingaccess holes 28 made in the segments 30 and 32. The hydrophone segment30 contains a connector 40 that may be removably connected to the cableconnector 17. The connectors 40 and 17 sealingly mate with each other toprevent fluid leakage into the conductors connected to such connectorsfor carrying electrical signals.

The hydrophone segments 30 and/or 32 may contain a preamplifier 45suitably coupled between the connector 17 and an active element of thehydrophone (described later) to amplify signals received from thehydrophone segments 30 and 32 prior to transmitting such signals throughthe cable 10. Hydrophone segments 30 and 32 are electrically coupled toeach other by suitable means known in the art, such as connectors.

The nose 34 preferably has two halves 34a and 34b, each such half havingan inner surface that substantially conforms to the cable outer surface,are placed against the hydrophone segments 30 and 32 and securelyattached to the cable 10 by a suitable means such as bolts. The outersurface of the nose 34 preferably has a smooth surface 39 extending fromthe cable diameter to the diameter of the hydrophone segments at the end38. The hydrophone segments 30 and 32 may be easily removed from thecable 10 by removing the nose section 34 and the bolts 26 to repair orreplace a particular hydrophone segment. Alternatively, the nose 33(shown in dashed lines) is comprised of a single section having an innersurface that substantially conforms to the cable outer surface, and isplaced against the hydrophone segments 30 and 32. The nose 33 issecurely attached to the cable 10 and hydrophone segments 30 and 32using the screw threads 35 on the nose 33 which attach to suitablematching screw threads 35 (shown in dashed lines) on the hydrophonesegments 30 and 32. A suitable attachment means, such as bolts, mayadditionally be used.

The above-described cable hydrophone utilizes two hydrophone segmentsattached around a cable. One segment contains a preamplifier and aconnector electrically coupling the hydrophone to a conductor take-outfrom the cable. Although the cable hydrophone described herein has twosegments, the hydrophone according to the present invention, however,may contain one or more segments that are electrically coupled to eachother. In certain applications, it may be desirable to utilize more thantwo hydrophone segments placed around the cable. In such cases,provision is made for placing the desired number of segments around thecable in a manner similar to that described above. The elements andconstruction of the hydrophone segments will now be described whilereferring to FIGS. 1-6.

FIG. 4 shows a plan view of the hydrophone segment 30 shown in FIG. 1and FIGS. 5A-5C show cross-sectional view taken along B--B of thehydrophone segment 30 shown in FIG. 4.

FIGS. 1A and 5A show a first embodiment of the hydrophone of the presentinvention including a compliant diaphragm 68 having a standoff ledge 69and made from a suitable polymer material, such as polycarbonate. Thediaphragm 68 having the standoff ledge 69 is more clearly shown in FIG.3. The hydrophone segment contains a mandrel or a substrate 50 having aninner surface 52 that preferably conforms to the surface on which suchsegment is intended to be mounted. In the case of the cable hydrophoneshown in FIG. 1, the substrate 50 has a concave inner surface thatconforms to the outer surface of a rigid member mounted on the outersurface of the cable 10.

The diaphragm 68 is place on the outer surface 54 of the substrate 50 todefine an enclosed chamber 66 between the diaphragm 68 and standoffledge 69 and the outer surface 54 of the substrate 50. The size of thediaphragm 68 is sufficient to cover the entire outer surface 54 of thesubstrate 50. A rectangular chamber 66 is preferred for use in the cablehydrophones. The depth of the chamber 66 depends upon the desiredapplication. The diaphragm 68 is hermetically sealed at the standoffledge 69 by a suitable means, such as an adhesive or a clamp, placedaround the entire periphery of the standoff ledge 69 to form a sealedspace or chamber 66 between the diaphragm 68 and the outer surface 54 ofthe substrate 50. The use of the standoff ledge 69 on the diaphragm 68allows for the use of un-machined tubing for manufacturing the substrate50. Forming the standoff ledge 69 into the molded diaphragm 68 providesa cost efficient method of manufacturing and assembling of thehydrophone.

FIGS. 1B and 5B show a second embodiment of the hydrophone of thepresent invention including a first substrate 49 and a second substrate50 having a concavity 56 on an outer surface 54. The first substrate 49has a concave inner surface that conforms to the outer surface of therigid member mounted on the outer surface of the cable 10 and the secondsubstrate 50 has a concave inner surface that conforms to the outersurface of the first substrate 49. The outer surface 54 of the secondsubstrate 50 has a concavity 56 of a desired shape and depth bounded byshoulders 58. A rectangular concavity 56 is preferred for use in thecable hydrophones. The depth of the cavity 56 depends upon the desiredapplication.

A damping compound or damping material 51, such as a polyurethane, orother suitable elastomeric compound, is placed between the outer surfaceof the first substrate 49 and the inner surface of the second substrate50 to isolate energy present in the cable 10 and provide for vibrationdamping. Preferably, the first substrate 49 is made of an aluminummaterial and the second substrate 50 of a carbon fiber material.However, both substrates can be made of aluminum material, carbon fibermaterial, or other suitable stiff material. Preferably, the firstsubstrate 49 is thicker in size than the second substrate 50.Alternatively, one or more additional substrates are used including adamping material placed between each additional substrate to providefurther vibration damping.

The concavity 56 is preferably covered by a compliant diaphragm 68 madefrom a suitable polymer material such as polycarbonate. The size of thediaphragm 68 is sufficient to cover the entire concavity 56. Thediaphragm 68 is hermetically sealed at the shoulders 58 by a suitablemeans 64, such as an adhesive or a clamp, placed around the entireperiphery of the concavity 56 to form a sealed chamber or space 66between the diaphragm 68 and the inner surface of the concavity 56. Thediaphragm 68 shown in FIGS. 1B and 5B is shown without a standoff ledge,however, alternatively, a diaphragm having a standoff ledge may be usedwith the substrate having a concavity to form a sealed chamber.

FIGS. 1C and 5C show a third and preferred embodiment of the hydrophoneof the present invention wherein the diaphragm 68 having the standoffledge 69 (shown in greater detail in FIG. 3) is used in a hydrophonehaving at least two substrates, shown as first substrate 49 and secondsubstrate 50. The diaphragm 68 is hermetically sealed at the standoffledge 69 by a suitable means, such as an adhesive or a clamp, placedaround the entire periphery of the standoff ledge 69 to form a sealedspace or chamber 66 between the diaphragm 68 and an outer surface 54 ofthe second substrate 50.

The first substrate 49 has a concave inner surface that conforms to theouter surface of the rigid member mounted on the outer surface of thecable 10 and the second substrate 50 has a concave inner surface thatconforms to the outer surface of the first substrate 49. The dampingmaterial 51 is placed between the outer surface of the first substrate49 and the inner surface of the second substrate 50 to isolate energypresent in the cable 10 and provide for vibration damping. Preferably,the first substrate 49 is made of an aluminum material and the secondsubstrate 50 of a carbon fiber material. However, both substrates can bemade of aluminum material, carbon fiber material, or other suitablestiff material. Preferably, the first substrate 49 is also thicker insize than the second substrate 50.

The sealed chamber 66 in each embodiment of the present inventionusually is filled with air or an inert gas such as nitrogen either atthe ambient pressure or at higher pressure. Also in each embodiment ofthe present invention, an active element 60 in the form of a thin sheetor film of a flexible resilient piezoelectric material, preferably apolyvinylidene fluoride ("PVDF") material, is placed over the diaphragm68. The active element 60 is preferably bonded to the diaphragm. Theactive member and the diaphragm flex in response to acoustic signals inthe form of pressure waves "P" (see FIG. 1). The thickness of the PVDFfilm 60 is preferably in the order of 28 microns, although otherthicknesses may be used. The diaphragm 68 provides spring action to theactive element 60 that defines response of the active element 60. Tomake the hydrophone segment, it may be desirable to first bond theactive member 60 to the diaphragm 68 and then bond the diaphragm tosubstrate 50. In an alternative embodiment, the active element 60 may beplaced directly on the concavity 56 shown in FIGS. 1B and 5B andhermetically sealed along the shoulders 58.

FIG. 6 shows a sectional view of the diaphragm 68 having the standoffledge 69 and including a plurality of convex ribs 53 longitudinallypositioned on an inner surface of the diaphragm 68. The plurality ofconvex ribs 53 is preferably included on the diaphragm 68 in eachembodiment of the present invention. The convex ribs 53 preserve theactive element 60 by controlling collapse of the diaphragm 68 underpressure and thereby preventing crumpling of the active element 60. Thedepth and size of the convex ribs 53 can be adjusted to provide varyingcontrol.

FIG. 7 shows the electrical connection takeout from the active member60. As shown in FIG. 7, a metalized electrode 70 is deposited over bothsides of most of the active element 60, leaving an inactive or inertborder or edge 72 around the electrode member 70. Evaporated silver orsilver ink are most common electrode compositions although other metals,such as gold, may be used. Electrical leads 74 and 76 deliver electricalsignals to signal conditioner or preamplifier 45. The signals from thesignal conditioner are passed to the connector 40 via the conductor 47.

Referring back to FIGS. 1-5, the active element 60 is covered by a layer80 of a suitable material. For cable hydrophone applications, the activeelement 60 and the substrate are preferably encapsulated by apolyurethane material, exposing only the necessary elements, such as theconnector 40, to the atmosphere while retaining the overall shape thatwill provide a snug fit of the assembly shown in FIG. 4 on the cable.Holes 28 that match the holes 24 in the flanges 22a and 22b are providedalong the edges of the segment 30 for attaching the segment to theflanges 22a and 22b as described earlier.

In operation, uniformly distributed radial acoustic hydrodynamictransient pressure fields, as represented by the inwardly directedarrows "P" such as shown in FIG. 1. exert compressive stress along thelongitudinal and lateral axes of the active element 60, generating avoltage output in response to pressure variations. Compensation forchanges in hydrostatic pressure is provided by the volume of gas in theenclosed chamber 66. As the external hydrostatic pressure increases ordecreases, inward contraction or expansion of the flexible resilientpiezoelectric element creates corresponding changes in the pressure ofthe gas chamber 66, thus equalizing the internal and external pressures.It has been found that for most of the applications in the field ofseismic exploration, a hydrophone having unpressurized air in thechamber produces adequate response.

The hydrophone segments 30 and 32 may be configured for optimumelectrical output at a desired range of operating depths by adjustingthe volumetric capacity of the chamber 66: a larger volume provides fora wider operating range. By reason of its construction, the hydrophoneis inherently insensitive to acceleration forces. A random localizedimpact, such as might be applied by a sharp object, will result in asmall signal, but the resulting electrical charge will be dissipatedover the entire active element 60 and become sufficiently attenuated soas be a virtually undetectable. The hydrophone, therefore, issubstantially electrically inert to localized mechanical forces. Becauseof the pressure equalization by the gas in the chamber 66, the activeelement is not sensitive to hydrostatic pressure variations.

Thus, the present invention provides a hydrophone having one or morehydrophone segments that are electrically coupled to provide a responsecorresponding to a hydrodynamic pressure applied to the segments. Eachsegment contains a chamber on an outer surface that is sealinglyenclosed by an active member made from a flexible, resilientpiezoelectric material. A single segment may be used as a hydrophone ora plurality of segments may suitably configured and electrically coupledto each other to form a hydrophone.

The foregoing description is directed to particular embodiments of thepresent invention for the purpose of illustration and explanation. Itwill be apparent, however, to one skilled in the art that manymodifications and changes to the embodiment set forth above are possiblewithout departing from the scope and the spirit of the invention. It isintended that the following claims be interpreted to embrace all suchmodifications and changes.

What is claimed is:
 1. An acoustic sensor comprising:(a) a substratehaving an outer surface; (b) a diaphragm having a standoff ledge andplaced on the outer surface of the substrate to define a sealed chamberbetween the diaphragm and the outer surface of the substrate; (c) apiezoelectric material placed over the diaphragm, wherein thepiezoelectric material is a sheet of a polyvinylidene fluoride material;and (d) a protective layer placed over the piezoelectric material. 2.The acoustic sensor as specified in claim 1 further having a connectorcoupled to the piezoelectric material.
 3. The acoustic sensor asspecified in claim 1, wherein the diaphragm and the piezoelectricmaterial are bonded to each other.
 4. The acoustic sensor as specifiedin claim 1, wherein the diaphragm includes a plurality of convex ribslongitudinally positioned on an inner surface.
 5. A sensor array,comprising:(a) a cable having a plurality of serially spaced take-outconductors along the length of the cable; (b) a separate hydrophoneplaced coupled to each said take-out conductor, each said hydrophonehaving two segments, each segment having:(i) a substrate having an outersurface and an inner surface adapted to be removably attached on thecable; (ii) a diaphragm having a standoff ledge and placed on the outersurface of the substrate to define a sealed chamber between thediaphragm and the outer surface of the substrate; (iii) a flexiblepiezoelectric material bonded to the diaphragm; and (iv) an insulatingmaterial placed on the piezoelectric material to seal the piezoelectricmaterial from the outside environment; and wherein the two segments aresecurely placed around the cable and electrically connected to eachother.
 6. The apparatus as defined in claim 5, wherein the flexiblepiezoelectric material is a sheet of a polyvinylidene fluoride material.7. The apparatus as defined in claim 5, wherein the sealed chambercontains a pressurized inert gas.
 8. The apparatus as specified in claim5, wherein the diaphragm includes a plurality of convex ribslongitudinally positioned on an inner surface.
 9. The apparatus asdefined in claim 5, wherein a rigid member is placed between thehydrophone and the cable outer surface.
 10. A hydrophone suitable fortowing under water by a cable, comprising:(a) an outer shape having anose section with a smoothly increasing cross-sectional diameter from apoint of attachment to the cable to a point of first outside diameterand a tail section with a smoothly decreasing cross-sectional diameterfrom a second outside diameter to a point of attachment to a tailbridle, said nose section having a slope that is greater than the slopeof the tail section; and (b) a hydrophone having two segments detachablyplaced between the points of the first and second outer diameters, eachsaid segment having:(i) a substrate having an outer surface; (ii) adiaphragm having a standoff ledge and placed on the outer surface of thesubstrate to define an enclosed chamber therebetween; (iii) apiezoelectric material placed over the diaphragm; and (iv) anencapsulating material placed on the piezoelectric material to protectthe piezoelectric material from the surrounding environment.
 11. Anacoustic sensor comprising:(a) a first substrate having an outersurface; (b) a second substrate having an inner surface and a concavityon an outer surface; (c) a damping material placed between the outersurface of the first substrate and the inner surface of the secondsubstrate; (d) a piezoelectric material placed over the concavity of thesecond substrate to form an enclosed chamber between the piezoelectricmaterial and the concavity; and (d) an encapsulating material placed onthe on the piezoelectric material to protect the piezoelectric materialfrom the surrounding environment.
 12. The acoustic sensor as specifiedin claim 11, wherein the piezoelectric material is a polyvinylidenefluoride material.
 13. The acoustic sensor as specified in claim 11,further having a connector coupled to the piezoelectric material.
 14. Anacoustic sensor as specified in claim 11, further having a diaphragmbetween the concavity and the piezoelectric material.
 15. The acousticsensor as specified in claim 14, wherein the diaphragm and thepiezoelectric material are bonded to each other.
 16. The acoustic sensoras specified in claim 14, wherein the diaphragm includes a plurality ofconvex ribs longitudinally positioned on an inner surface.
 17. Theacoustic sensor as specified in claim 11, further having one or moreadditional substrates wherein the damping material is placed betweensaid additional substrates.
 18. A sensor array, comprising:(a) a cablehaving a plurality of serially spaced take-out conductors along thelength of the cable; (b) a separate hydrophone placed coupled to eachsaid take-out conductors, each said hydrophone having two segments, eachsegment having:(i) a first substrate having an outer surface and aninner surface, said inner surface adapted to be removably attached onthe cable; (ii) a second substrate having an outer surface with aconcavity thereon and an inner surface; (iii) a damping material placedbetween the outer surface of the first substrate and the inner surfaceof the second substrate; (iv) a flexible piezoelectric material placedon the concavity of the second substrate to form an enclosed chamber;and (v) an insulating material placed on the piezoelectric material toseal the piezoelectric material from the outside environment; andwherein the two segments are securely placed around the cable andelectrically connected to each other.
 19. The apparatus as defined inclaim 18, wherein the flexible piezoelectric material is a sheet of apolyvinylidene fluoride material.
 20. The apparatus as defined in claim18, wherein the enclosed chamber contains a pressurized inert gas. 21.The apparatus as defined in claim 18, further having a diaphragm betweenthe concavity and the flexible piezoelectric material.
 22. The apparatusas specified in claim 21, wherein the piezoelectric material is bondedto the diaphragm.
 23. The apparatus as specified in claim 21, whereinthe diaphragm includes a plurality of convex ribs longitudinallypositioned on an inner surface.
 24. The apparatus as defined in claim18, wherein a rigid member is placed between the hydrophone and thecable outer surface.
 25. The apparatus as specified in claim 18, furtherhaving one or more additional substrates wherein the damping material isplaced between said additional substrates.
 26. A hydrophone suitable fortowing under water by a cable, comprising:(a) an outer shape having anose section with a smoothly increasing cross-sectional diameter from apoint of attachment to the cable to a point of first outside diameterand a tail section with a smoothly decreasing cross-sectional diameterfrom a second outside diameter to a point of attachment to a tailbridle, said nose section having a slope that is greater than the slopeof the tail section; and (b) a hydrophone having two segments detachablyplaced between the points of the first and second outer diameters, eachsaid segment having:(i) a first substrate having an outer surface and aninner surface, said inner surface adapted to be removably attached onthe cable; (ii) a second substrate having an outer surface with aconcavity thereon and an inner surface; (iii) a damping material placedbetween the outer surface of the first substrate and the inner surfaceof the second substrate; (iv) a diaphragm placed on the concavity of thesecond substrate to form an enclosed chamber therebetween; (v) apiezoelectric material placed over the diaphragm; and (vi) anencapsulating material placed on the piezoelectric material to protectthe piezoelectric material from the surrounding environment.
 27. Anacoustic sensor comprising:(a) a first substrate having an outersurface; (b) a second substrate having an inner surface and an outersurface; (c) a damping material placed between the outer surface of thefirst substrate and the inner surface of the second substrate; (d) adiaphragm having a standoff ledge and placed over the outer surface ofthe second substrate to define a sealed chamber between the diaphragmand the outer surface of the second substrate; (e) a piezoelectricmaterial placed over the diaphragm wherein the piezoelectric material isa sheet of a polyvinylidene fluoride material; and (g) a protectivelayer placed over the piezoelectric material.
 28. The acoustic sensor asspecified in claim 27, further having a connector coupled to thepiezoelectric material.
 29. The acoustic sensor as specified in claim27, wherein the diaphragm and the piezoelectric material are bonded toeach other.
 30. The acoustic sensor as specified in claim 27, furtherhaving one or more additional substrates wherein the damping material isplaced between said additional substrates.
 31. The acoustic sensor asspecified in claim 27, wherein the diaphragm includes a plurality ofconvex ribs longitudinally positioned on an inner surface.
 32. A sensorarray, comprising:(a) a cable having a plurality of serially spacedtake-out conductors along the length of the cable; (b) a separatehydrophone placed coupled to each said take-out conductor, each saidhydrophone having two segments, each segment having:(i) a firstsubstrate having an outer surface and an inner surface, said innersurface adapted to be removably attached on the cable; (ii) a secondsubstrate having an outer surface and an inner surface; (iii) a dampingmaterial placed between the outer surface of the first substrate and theinner surface of the second substrate; (iv) a diaphragm having astandoff ledge and placed on the outer surface of the second substrateto form an enclosed chamber between the diaphragm and the outer surfaceof the second substrate; (v) a flexible piezoelectric material bonded tothe diaphragm; and (vi) an insulating material placed on thepiezoelectric material to seal the piezoelectric material from theoutside environment; and wherein the two segments are securely placedaround the cable and electrically connected to each other.
 33. Theapparatus as defined in claim 32, wherein the flexible piezoelectricmaterial is a sheet of a polyvinylidene fluoride material.
 34. Theapparatus as defined in claim 32, wherein the enclosed chamber containsa pressurized inert gas.
 35. The apparatus as defined in claim 32,wherein a rigid member is placed between the hydrophone and the cableouter surface.
 36. The apparatus as specified in claim 32, wherein thediaphragm includes a plurality of convex ribs longitudinally positionedon an inner surface.
 37. The apparatus as specified in claim 32, furtherhaving one or more additional substrates wherein the damping material isplaced between said additional substrates.
 38. A hydrophone suitable fortowing under water by a cable, comprising:(a) an outer shape having anose section with a smoothly increasing cross-sectional diameter from apoint of attachment to the cable to a point of first outside diameterand a tail section with a smoothly decreasing cross-sectional diameterfrom a second outside diameter to a point of attachment to a tailbridle, said nose section having a slope that is greater than the slopeof the tail section; and (b) a hydrophone having two segments detachablyplaced between the points of the first and second outer diameters, eachsaid segment having:(i) a first substrate having an outer surface; (ii)a second substrate having an outer surface and an inner surface; (iii) adamping material placed between the outer surface of the first substrateand the inner surface of the second substrate; (iv) a diaphragm having astandoff ledge and placed on the outer surface of the second substrateto define an enclosed chamber therebetween; (v) a piezoelectric materialplaced over the diaphragm; and (vi) an encapsulating material placed onthe piezoelectric material to protect the piezoelectric material fromthe surrounding environment.